Properties Of Gases And Diffusion

Question 1

Give an overview of the kinetic theory of gases

Answer 1

• gases are a collection of molecules moving randomly around a space
• Pressure is generated by collisions of molecules with a surface
• the more frequent and harder the collisions the higher the pressure generated by the gas
• Properties of gases can be described by the ideal gas equation, pV=nRT

Question 2

What is Boyles law

Answer 2

Pressure (P) of a gas is inversely proportional to its volume (V) if temperature & number of gas molecules remains constant in a closed system
P is proportional to 1/V
Mechanism of inspiration & expiration is an example of Boyle’s Law

Question 3

What is the partial pressure of a gas

Answer 3

In a mixture of gases, the total pressure = the sum of the partial pressures of the individual gases.

Partial pressure. Eg in a mix of 1/3 O2 and 2/3 N2
Partial pressure of O2 = 1/3 of Total pressure
Partial pressure of N2 = 2/3 of Total pressure
Total pressure = Partial pressure of O2 + Partial pressure of N2

Question 4

What is atmospheric pressure

Answer 4

• pressure exerted by the weight of the air above the earth in the atmosphere
• At sea level: 101 kilopascals (kPa) = 1 atmosphere = 760 mmHg
• At high altitudes atmospheric pressure is lower (weight of air pressing down is less)

Question 5

What is the composition of atmospheric air

Answer 5

Total Atmospheric Pressure at sea level = 101 kiloPascals
Partial pressure of O2 = 101 x 20.9% = 21.1 kPa
Partial pressure of N2 = 101 x 78 % = 78.7 kPa
Partial pressure of CO2 = 101 x 0.03%= 0.03 kPa

Question 6

How do gasses diffuse

Answer 6

Gases dissolve and diffuse according to their partial pressure
• In the body gases diffuse down their partial pressure gradient
• from area of high partial pressure – to low partial pressure e.g. movement of oxygen from alveolar air ↔ blood
• Partial pressures (rather than concentrations) used to describe gases in the body
• Denoted by ‘p’ - as in pO2

Question 7

What happens when inspired gases come in intact with body fluid

Answer 7

• Gas molecules will enter water to dissolve in liquid

* Water molecules evaporate to enter air

Question 8

What happens at equilibrium in terms of vapour pressure of water

Answer 8

• Water molecules entering the air exert ‘vapour pressure’. At equilibrium,
• rate of H2O evaporation = rate of H2O condensation
• the air is saturated with vapour
• Saturated Vapour Pressure (SVP) = 6.28kPa at body temp.
• Inhaled air becomes saturated with water, in the upper respiratory tract

Question 9

Describe the partial pressure of gases in moose air

Answer 9

How does the water vapour affect the partial pressure of the other gases?
• Saturated vapour pressure = 6.28 kPa at body temperature
• Pressure of the rest of the gases = 101 – 6.28 = 94.7 kiloPascals
• They are still in the same ratios as in dry air
So pO2 = (101 - 6.28) x 20.9% = 19.8kPa
pN2= (101 - 6.28) x 78 % = 73.8 kPa
pCO2=(101 - 6.28) x 0.03%= 0.03kPa

Question 10

What happens at equilibrium in terms of gases dissolving in body fluids

Answer 10

Gas dissolves in body fluids
• Dissolved gas molecules also exert pressure in the liquid
• Equilibrium is reached when: rate of gas entering water = rate of gas leaving the water.
At equilibrium,
• the partial pressure of the gas in the liquid = partial pressure of the gas in the air above it

Question 11

Give a summary of gases in liquid

Answer 11

• Partial pressure is the pressure exerted by the dissolved gas in the
liquid At equilibrium, • the partial pressure of the dissolved gas in the liquid = the partial
pressure of the gas it is exposed to
• Note: another term used for partial pressure of a gas in the liquid is
‘tension’ (e.g. oxygen tension in blood)

Question 12

How is partial pressure different to the amount of a dissolved gas

Answer 12

Amount dissolved = partial pressure x solubility coefficient of gas
(Solubility coefficient – is a constant for the individual gas and the solvent)
Solubility coefficient of O2 in plasma = 0.01 mmol/ L /kPa (at 37°C) o
So When exposed to a pO of 13.3 kPa (as in alveolar air)
- 0.01 x 13.3 = 0.13 mmol of O2 will dissolve
Plasma has 0.13 mmol dissolved oxygen /per litre. 0.01 x 13.3 = 0.13

Question 13

What happens if a gas reacts to a component of the liquid in Addison to dissolving and give an example

Answer 13

If a gas reacts (e.g. O component of the liquid in addition to dissolving, this reaction must complete before equilibrium is reached and partial pressure is established.

• O2 enters plasma & dissolves in it
• dissolved O2 enters RBC to bind to Hb
• Process continues till Hb fully saturated
(each Hb molecules binds 4 O2 molecules)
• after Hb is fully saturated, O2
dissolve till equilibrium is reached
• At equilibrium, pO2 of plasma = alveolar air

Question 14

How is oxygen bound to Hb downloaded into tissues

Answer 14

• Blood contains both dissolved and Hb bound
oxygen
• The pO is a measure of dissolved O2 in the blood.
• Dissolved O2 is available to diffuse into tissues down its partial pressure gradient
• As dissolved O2 leaves the blood, it will be replaced
By O2 bound to Hb.
• In this way, the oxygen bound to Hb will bedownloaded and diffuse into tissues

Question 15

Define partial pressure

Answer 15

Partial pressure = pressure exerted by the dissolved gas in a liquid

Question 16

Define conc of a dissolved gas

Answer 16

Concentration of dissolved gas = number of mmol of gas, dissolved in a litre of liquid

Question 17

Define solubility coefficient

Answer 17

Solubility coefficient = a constant for each gas (solute) and solvent

Question 18

What is the total content of gas

Answer 18

Total content of gas = dissolved gas + gas bound to or reacted with a component

Question 19

How do the partial pressures of o2 and co2 in alveolar air comaore to in inhaled air

Answer 19

In alveolar air :
pO2=13.3 kPa (lower than inhaled air)
PCO2=5.3 kPa (higher than in inhaled air)
Because
• Inhaled air mixes with residual volume
• Effect of O2 diffusing across the alveolar wall
• effect of CO2 entering the alveoli

Alveolar air composition stays constant around this level; Blood equilibrates to this level

Question 20

Describe the gradient s in moved venous blood compared to alveolar air

Answer 20

• Alveolar PO2 > PO2 in mixed venous blood
• Alveolar PCO2 > PO2
in mixed venous blood
• so oxygen will diffuse into blood and carbon dioxide out

Question 21

What affects the rate of diffusion

Answer 21

• partial pressure difference (gradient) across membrane (P1-P2)
• A - the surface area available for diffusion
• T – (thickness) i.e. distance. molecules must diffuse
• Diffusion coefficient of the individual gas:
• The solubility of the gas in the liquid : greater the solubility, faster the rate of diffusion
• Molecular weight of gas:
– Higher the molecular weight slower the rate of diffusion
Diffusion coefficient (D) is proportional to solubility/ root molecular wt
• used to determine the relative rates at which different gases will diffuse across the same membrane at the same pressures;

Question 22

Compare the diffusion of CO2 vs O2

Answer 22

• Solubility: CO2 much more soluble than O2 (so diffuses faster)
• Molecular weight: molecular weight of CO2 > O2 (slows down CO2)

Combine the two factors Oxygen is small and thus fast, but CO2 is more soluble Overall, the effect of solubility is greater
CO 2 diffuses 20 times faster than O2
than CO
• Larger difference in partial pressures (ΔP) compensates for slower diffusion of O2
• In a diseased lung, O2 gas exchange is thus more impaired than CO2, O2 slower diffusion rate

Question 23

What are thelayers of diffusion barrier from air to rbc

Answer 23

• Fluid film lining alveolus
• epithelial cell of alveolus
• Interstitial space
• endothelial cell of capillary
• plasma
• red cell membrane
• 5 cell membranes
• 3 layers of cytoplasm
• 2 layers of tissue fluid +plasma

Question 24

Describe the surface o the alveolar capillary

Answer 24

• The surface area of the alveolar capillary membrane about 100 m2 • Barrier < 0.4 μM thick • oxygen exchange complete in 1/3 of time blood spends capillary • so plenty of reserve – for exercise

Question 25

What factors affect rate of gas diffusion in disease

Answer 25

• Thickness of the membrane
– Increase as a result of oedema fluid in the interstitial space and in alveoli
– Lung fibrosis - increased thickness of alveolar capillary membrane
• Surface area of the membrane
– decreased by removal of an entire lung
– Emphysema - decreased surface area
• Diffusion coefficient of the gas:
– CO2 always diffuses much faster than O2
– So, diffusion of O2 affected → pO2 is low
– Diffusion of CO2 not affected → pCO2 is normal

Question 26

What are probelms with alveolar capillary membrane

Answer 26

See slide